This is a U.S. national stage of application No. PCT/DE99/02971, filed on Sept. 14, 1999. Priority is claimed on that application and on the following application: Country: Germany, Application No.: 198 49 472.6, filed Oct. 21, 1998.
1. Field of the Invention
The invention relates to a plasticizing unit in injection-moulding machines, in particular for processing thermoplastics, with a plasticizing cylinder in which a rotatable and axially displaceable screw is arranged, at the head of which a non-return valve is provided, which has an axially displaceable barrier sleeve which encloses with radial play a tapered region which belongs to a basic body fixed in the screw head and is present between blades and a stop ring.
2. Description of the Related Art
During the injection-moulding of thermoplastics, screws which are axially displaceable in plasticizing cylinders and are rotatable in a defined direction are used. The polymer granules are thereby plasticized by the friction occurring as a result and possibly by heating elements additionally used. For injecting the polymer melt into the cavity of a mould, the screw is used as a ram and, for this purpose, is moved axially in the plasticizing cylinder. To prevent material which is to be plasticized from flowing back during this injection operation, that is to say when the non-rotating screw is advancing, non-return valves are used. These non-return valves have a barrier sleeve, which is pressed during injection against a thrust-ring surface of a basic body arranged at the head of the screw.
During metering, i.e. when the rotating screw is returning, the barrier ring is lifted off the end face, representing the thrust face, and pressed against the bearing surfaces of blades arranged at the head of the basic body. The barrier ring sympathetically rotates at a relatively slow speed as a result of the friction on the wall of the plasticizing cylinder during the rotation of the screw while metering is taking place. This causes dry running between the bearing surfaces of the blades and the barrier sleeve and consequently increased wear.
To reduce the wear, various measures have been proposed. For instance, it is known from DE 37 11 775 A1 to design the corresponding surfaces of the displaceable barrier sleeve and of the blades in such a way that the neighboring end faces form an angle of between 5 and 30xc2x0, expediently 15xc2x0. By this measure, it is intended during the metering operation, that is to say while the screw is rotating, for plasticized material to be drawn between the surfaces of the barrier sleeve and those of the blades, so that a constantly renewing film of material is maintained there, preventing dry running.
It is known from DE 297 01 495 U1 to grip the barrier sleeve or the stop ring positively during the metering operation and consequently make it rotate at the same speed as the screw itself in the plasticizing cylinder.
In the case of the disclosed plasticizing units in injection-moulding machines, the non-return valves used have a large amount of wear and/or there is local damage to the injection-moulding compound and the contact surface of the barrier sleeve in relation to the blades or the plasticizing cylinder.
The invention was based on the object of providing a plasticizing unit in injection-moulding machines which, with a simple construction, has a non-return valve which ensures the greatest possible functional reliability with a defined small amount of wear and minimal damage to the polymer material.
According to the invention, supporting elements are provided on the blades of the basic body, with at least the supporting elements being made of a hard-wearing material. If at least three supporting elements are used, the barrier sleeve is supported uniformly on these hard-wearing supporting surfaces during the metering operation.
The constant supporting surfaces, acting like three sliding bearings, have the effect that the frictional force in the direction of the contact surface of the barrier sleeve is calculable and constant during the metering operation.
Furthermore, measures are provided to ensure during the metering operation, that is to say when the screw is rotating and there is relative movement between the blades of the basic body and the barrier sleeve, that the polymer material located here is influenced as little as possible, in particular to prevent damage to the raw material.
In one case, the head ends of the supporting elements protrude beyond the surfaces of the blades. In the other configuration, the surface of the blades facing the barrier sleeve is of an arcuate design. In both cases, a sliding movement between the surface of the blades and the surface of the barrier sleeve is avoided, so that streaking or discoloration of the moulded part cannot occur.
In an advantageous configuration, the supporting elements are designed as pins and are arranged at right angles in relation to the supporting surface of the barrier sleeve. Furthermore, the pins are conically widened in the direction of the barrier sleeve, so that they can be pressed into the blades in a self-securing manner. The pins are in this case made of hard metal or ceramic or have, at least in the region of the supporting surface, a layer which contains carbides or borides.
In a further configuration, in particular when more than three blades are used, force elements which ensure that the surfaces of the pins are pressed uniformly against the supporting surface of the barrier sleeve are arranged at the foot end of the pins.
The barrier sleeve may in this case be designed in such a way that a small amount of wear is still accepted, especially since the barrier sleeve is a relatively simple geometrical part which can be easily exchanged. In a further configuration, the bearing surface in relation to the supporting elements is likewise made of hard-wearing material, so that consequently no wear occurs between the basic body and the barrier sleeve. In special cases, the entire barrier sleeve may be produced from a wear-reducing material.